Montelukast sodium namely Sodium 1-[[[(1R)-1-[3-[(1E)-2-(7-chloro-2-quinolinyl)ethenyl]phenyl]-3-[2-(1-hydroxy-1-methylethyl)phenyl]propyl]thio]methyl]cyclo propane acetic acid has the formula

Montelukast sodium is a leukotriene antagonist and inhibits the synthesis of leukotriene biosynthesis. It is useful as anti-asthmatic, anti-allergic, anti-inflammatory, cytoprotective agent and hence useful in the treatment of angina, cerebral spasm, glomerular nephritis, hepatic, end LS toxemia, uveitis and allograft rejection.
EP 0 480 717 discloses Montelukast sodium along with other related compounds and the methods for their preparation. The reported method of synthesis proceeds through corresponding methyl ester namely, and involves coupling methyl 1-(mercaptomethyl)cyclopropane acetate with a mesylate generated in-situ. The methyl ester is hydrolyzed to free acids and the latter to converted directly to Montelukast sodium salt. The process is not suitable for large-scale production because it requires tedious chromatographic purification of the methyl ester intermediate and for the final product with low yield.
U.S. Pat. No. 5,614,632 discloses a process for the preparation of the sodium salt of montelukast and certain process intermediates. The process involves generation of dilithium dianion of 1-(mercaptomethyl)cyclopropaneacetic acid followed by condensation with 2-(2-(3(S)-(3-(2-(7-chloro-2-quinolinyl)ethenyl)phenyl)-3-methanesulfonyloxypropyl)phenyl)-2-propanol (referred as mesylated alcohol) to afford montelukast, which is further converted to the corresponding sodium salt via dicyclohexyl amine salt. The '362 patent also discloses a process for the preparation of crystalline montelukast sodium salt and mesylated alcohol. The process involves reacting methyl 2(3(S)-(3-(2-(7-chloro-2-quinolinyl)ethenyl)phenyl)-3-hydroxy propyl)benzoate with methyl magnesium chloride to give a diol, which is further converted to mesylated alcohol on reaction with methane sulfonyl chloride. While certain processes of its preparation are known, there is a continuing need for new processes of preparation of montelukast and its salts. It is mentioned by the inventors of U.S. Pat. No. 5,614,632, that the crystalline montelukast dicyclohexylamine salt offers an efficient method for the purification of montelukast, which circumvents the need to use chromatographic purification.
Processes for preparation of montelukast and its intermediates have also been described in U.S. Pat. No. 5,523,477, and U.S. Patent Application Publication Nos. 2005/0234241, 2005/0256156, 2005/0107612, and International Application Publication Nos. WO 2005/105749, WO 2005/000807, WO 2004/108679, and WO 2006/021974.
WO 2006/08751 discloses a process for preparation of Montelukast sodium by neutralizing Montelukast organic amine salts such as [alpha]-Methyl benzyl amine salt, diisopropyl amine salt, dibenzyl amine salt followed by treatment with ethanolic sodium hydroxide.
Although many processes have been described in the prior art for the preparation of montelukast and its intermediates, there still remains a need for a process for the preparation of montelukast which is industrially viable.